ACA Mini Listening Impressions

Third post from a novice builder of the Nelson Pass designed ACA Mini amplifier. My amp is built and biased, and I've spent some hours listening to it. My impressions follow the photo of my completed amp.

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I have listened to the ACA Mini, well warmed up, in two systems, and now I venture to offer some impressions.

System 1 (cobbled together): WiiM Mini streamer, Schiit Modi 3 DAC, Schiit SYS (volume control), ACA Mini, PSB Alpha T20 speakers (small floorstanders, about 87 db sensitivity).

System 2 (main living room system): WiiM Pro streamer, Denafrips Ares II DAC, Schiit Saga+ hybrid preamp with a 1946 RCA gray glass 6SN7GT tube, ACA Mini (in place of Schiit Aegir), JBL Arena 170 speakers (somewhat larger floorstanders, stated sensitivity of 86 db [I think], but really more efficient than the PSBs).

Impressions: The ACA Mini has incredible clarity in the treble and high mids, with a fantastic soundstage. This is where it really stands out. What it lacks is heft in the lower midrange and bass. My thought is that it has a good skeleton but could use more muscle.

The tilt toward the treble at the expense of lower mids is particularly noticeable with vocals and instruments like acoustic guitars, where the fundamentals are in the lower midrange.

Also lacking at the low end is a degree of impact necessary for a realistic piano left hand, not to mention drums.

By contrast, the Aegir excels with lower mids and bass. It has very detailed treble, but here I could give the nod to the ACA Mini.

If my Aegir was out for service, I could easily enjoy music with the ACA Mini in its place. Not bad!

Thanks to Nelson Pass for offering this kit to us. I had a ball building it. Honestly, I felt like a teenager, learning, doing, and having fun.
 
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try with no jumper and give a pair day as all is new....
jbl have some problem on bass , whathifi:That lack of great room-shaking ability is partly down to the JBL Arena 170s’ bass performance: what’s there is fairly detailed and agile, but the fact it lacks power and punch makes the presentation feel just a little lacking in heft. Big crescendos aren’t as imposing or as impressive as they should be."
I have great low end in my scanspeak
 
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Hi, @nicoch58.

Of course I will give the ACA Mini time to burn in. It will be in my main system for a long time. I am currently listening at a low level to Grant Green's "Idle Moments," and it sounds wonderful, especially the vibraphone.

These cheap JBL Arena 170s are surprisingly good. I particularly like their lower midrange. For the lowest bass I use an RSL Speedwoofer 10S Mk2. At the moment, though, the subwoofer is off.

I don't trust WhatHiFi reviews. For one thing, they change their reviews after they are posted. For another, they are wrong about very basic things, like placing the Arena 170s near the back wall. They sound best three feet from the back wall. Finally, they exist to push British brands. I bought a Cambridge Audio CXA60 based partly on their review. Bad amp: hum that they couldn't fix and a treble glare I could not get used to. I gave it away. (Oh, well. We learn from our mistakes.)
 
Hi again, @nicoch58.

Your latest post puzzles me. If you are referring to the JBL Arena 170 speakers, they have a "7 inch" woofer, but the cone diameter is only 4 1/4 inches. I don't expect much bass from that, though the big enclosures help. What I do notice is that these JBLs have a convincing lower midrange, so an instrument like a trombone sounds right. My PSB Alpha T20 speakers have two (tiny) "5 1/4 inch" mid/woofers and simply cannot render a trombone properly.

The JBLs are 8 Ohms, as are the PSBs. I shy away from lower impedances.
 
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Hi, @Mikerodrig27.

I suspected that the sound of the ACA Mini would improve with more efficient speakers, but the JBLs are the best I can do. They seem to be pretty easy to drive, and I listen at low to moderate levels.

I experimented with the jumpers last night, when I had the Mini driving PSB Alpha T20 speakers. These have aluminum tweeters that aren't over-bright, but not as clean sounding as the soft domes in the JBLs. With the PSBs, the treble seemed harsh and distorted when I removed the jumpers. [I forgot to say this to @nicoch58.]

I'll experiment with the jumpers again with the JBLs. I plan to leave the ACA Mini in my main system for quite a while. It has some traits, notably the treble detail and incredible soundstage, that I want to keep hearing. Maybe with more burn-in the lower mids and bass will improve, or maybe I'll get used to the ACA Mini sound and think it's normal.

Up to now, I have never heard an amp designed by Nelson Pass. The ACA Mini gives me an idea of how great his larger amps must sound. Very nice!
 
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Pass's amps have all kinds of sounds. If you lust after a big soundstage, Alephs throw off a big rich sound. Although, I am sure the ACA Mini will keep you busy listening to music for quite a while.

Let us know if your listening impressions change as the amp and/or you burn in :)
 
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Update: I tried the ACA Mini with the jumpers removed in my main system (JBL Arena 170 speakers) and the treble is distorted, just like with the PSB Alpha T20s. The trumpet in Art Blakey's "Night in Tunisia" album (Blue Note) sounded pretty bad. So, keeping the jumpers installed.

One thing that comes across very clearly with the title track on "Night in Tunisia" is the grunting/speaking in the background from the musicians. The ACA Mini really excels in the details department.

I noticed today that there is a long thread for the ACA Mini. Maybe I should have posted there.
 
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Maybe the ACA is just less forgiving? I just listened to Night in Tunisia from the album Night and Tusnia to see if I hear what you are saying. I have AMT tweeters, a topping D90 (which is not forgiving at all) no preamp. The highs in that song are pretty harsh sounding. I think with a warmer sonic signature like how the Schiit Aegir is claimed to have, the song would be much more enjoyable.

My impressions were that I could hear more detail with the jumper out. But I could see how some people would appreciate the jumper in.

Art Blakely's Moanin' is a song I really like. Lee Morgan killed the trumpet solo in that song.
 
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Hi again, @Mikerodrig27.

"Moanin," written by the piano player Bobby Timmons, is a favorite of mine too. Lee Morgan shines on the studio version, but on the live album, "First Flight to Tokyo," he seems not to know where he wants to go with his solo. Such is the nature of improvisation. See what you think.

I believe you are right about the Aegir having a warm sound. It's not very forgiving of a so-so DAC like my Schiit Modi 3+, though. I bought a Denafrips Ares II because I got listening fatigue with the Modi 3+.

Incidentally, I prefer vinyl records, but I use my DAC a lot, in order to listen to a variety of music.

I just finished playing "Claude Debussy Plays His Finest Works" on Amazon Music. These are piano rolls transferred to digital. It is a hauntingly beautiful album, both because of the music and the fact that the composer is playing it. The ACA Mini really reveals the airy, romantic ("impressonistic" some say) quality of these pieces. However, I greatly miss the lower mids and bass (still). When I play this in the living room using the Aegir and walk into another room, it sounds like someone is playing a piano in the living room. With the ACA Mini, it sounds more like someone is playing a (very good) radio.
 
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Looking at the specs, the Schiit Aegir is rated at 20W into 8 Ohm with less than 0.01 % THD.

The ACA Mini is rated at about 5W into 8 Ohm with approximately 0.9 % THD with no jumper and 0.5 % THD with jumper.

So if anything, the ACA Mini would be perceived as "warmer". However if the ACA Mini is pushed hard and at its power limit with low sensitivity speakers, the distortion level becomes too high and then it could sound harsh.

From my experience, lower distortion gives the impression of more detail.
 
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Hi, @Ben Mah.

I don't think I'm pushing the ACA Mini too much. I listen with about 80 db peaks, meaning I only go beyond the "first watt" for transients too brief to measure.

I think I'm calling the Aegir warmer because it has a fuller sound and probably rolls off the treble a tiny bit.

Whatever the cause, there is no doubt in my mind that the ACA Mini reveals more detail than the Aegir. It's wonderful!
 
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Hi again, @Mikerodrig27.

"Moanin," written by the piano player Bobby Timmons, is a favorite of mine too. Lee Morgan shines on the studio version, but on the live album, "First Flight to Tokyo," he seems not to know where he wants to go with his solo. Such is the nature of improvisation. See what you think.

I believe you are right about the Aegir having a warm sound. It's not very forgiving of a so-so DAC like my Schiit Modi 3+, though. I bought a Denafrips Ares II because I got listening fatigue with the Modi 3+.

Incidentally, I prefer vinyl records, but I use my DAC a lot, in order to listen to a variety of music.

I just finished playing "Claude Debussy Plays His Finest Works" on Amazon Music. These are piano rolls transferred to digital. It is a hauntingly beautiful album, both because of the music and the fact that the composer is playing it. The ACA Mini really reveals the airy, romantic ("impressonistic" some say) quality of these pieces. However, I greatly miss the lower mids and bass (still). When I play this in the living room using the Aegir and walk into another room, it sounds like someone is playing a piano in the living room. With the ACA Mini, it sounds more like someone is playing a (very good) radio
I agree. The energy in that live version was not the same. In the studio version, his trumpet really tells a story. The way each phrase plays into the next is really nice. The live version you mentioned did not tie together as well.

I have been wanting to hear a Denafrips DAC. At some point, I want to try a R2R or a PCM63/PCM1704 DAC. Probably soon. Maybe even build one.

I have only played around with vinyl a little bit. I have a Pioneer PL530 that I went through and restored. New caps, Dynavector cartridge, new veneer etc. I need to pull it out and use it.

"Claude Debussy Plays His Finest Works" - I added this to my list of things to play over the next few days. :)
 
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Looking at the specs, the Schiit Aegir is rated at 20W into 8 Ohm with less than 0.01 % THD.

The ACA Mini is rated at about 5W into 8 Ohm with approximately 0.9 % THD with no jumper and 0.5 % THD with jumper.

So if anything, the ACA Mini would be perceived as "warmer". However if the ACA Mini is pushed hard and at its power limit with low sensitivity speakers, the distortion level becomes too high and then it could sound harsh.

From my experience, lower distortion gives the impression of more detail.
That is a good point. Although distortion hasn't always directly correlated with warmth in my experience. In most cases yes but not all cases.
 
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.....................

I just finished playing "Claude Debussy Plays His Finest Works" on Amazon Music............................................................................................................................................The ACA Mini really reveals the airy, romantic ("impressonistic" some say) quality of these pieces. However, I greatly miss the lower mids and bass (still). When I play this in the living room using the Aegir and walk into another room, it sounds like someone is playing a piano in the living room. With the ACA Mini, it sounds more like someone is playing a (very good) radio.
The Aegir with lower distortion would have a more detailed high end and sound more live. That has been my experience as I have been reducing the distortion in my system and as the distortion decreased, I found that details in the higher frequencies became more apparent and the sound seemed more realistic and live.
 
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